I get to 2-4 Seahawks games a year and go through the same type of thing... trying to get good seats at good prices, etc.
Ticket sales typically start in late July and you'll get nosebleeds if they don't sell out.
I have had good luck on e-bay, but here is the best advise I can give you...
Although it is always more comfortable showing up at the stadium with tickets in hand, there are always scalpers selling tickets. In my experience, you can get them darn close to face value. It's quite a racket, though, and you can expect to get an education if you've never been through it before. I was haggling with some scalpers last year and after we got what we wanted my daughter said, "well, that was interesting!"
I think the kids were just a little bit intimidated.